1. Field of the Invention
The present invention generally relates to a multiple data communication system used in an automobile. More specifically, the present invention is directed to an automobile multiple communication system employing a parent station and a plurality of child stations, capable of preventing power consumption such as a dark current of a battery in these child stations.
2. Description of the Prior Art
Recently, various electronic components such as speed sensors and touch switch sensors have been normally utilized in automobiles. Under such circumstances, an amount of connecting wires, i.e., wire harness for electrically connecting a large number of terminal units with a large quantity of input units is considerably increased, as compared with that of the old-fashioned automobiles. The terminal units are, for instance, headlights, sidemarker lamps and actuators, whereas the input units are, for example, switches and sensors. Since the total amount of the wire harness is considerably increased, there are problems that the total weight of the automobile is increased and reliabilities of these unit assemblies are deteriorated.
To solve the above-described problems, various types of multiplex communication systems have been developed, by which various information about drive controls for a large quantity of terminal units can be firmly transferred via a small number of connecting wires to the input units.
In one conventional multiplex communication system, there are provided child stations at the terminal side, to which a plurality of terminal units are connected, and also a parent station at the input side, to which a plurality of input units are connected. These plural child stations and parent station are mutually connected via a common communication line. Then, the operation data about the input units are acquired and the relevant terminal unit can be driven by performing the multiplex data communication operation under control of the communication control microcomputers employed in these child stations and parent stations. Such a conventional multiplex communication system for automobiles is described in, for instance, Japanese Laid-open (KOKAI DISCLOSURE) Patent Application No. 3-25046 opened on Feb. 1, 1992.
In accordance with this conventional multiplex automobile communication system, the parent station judges whether the present communication mode corresponds to the normal communication mode, or the low-power-consumption communication mode. In the normal communication mode, the ignition switch is turned ON, whereas in the low-power-consumption communication mode, the ignition switch is turned OFF. When the parent station judges that the present communication mode is "the normal communication mode", these child stations are set to the normal operation conditions (will be referred to as "wake conditions"), and subsequently acquire various information derived from the child stations, and furthermore controls the child stations based upon the acquired information. To the contrary, when the parent station judges that the present communication mode is the low-power-consumption mode, the parent station sends information for instructing the low power consumption operation (will be referred to as "sleep operation") to the child station. As a result, after the respective child stations have been set to the low-power-consumption conditions (will be referred to as "sleep conditions") by the parent station, this parent station itself is brought into the "sleep" condition.
It should be noted in this conventional multiplex communication system that when all of the child stations are turned ON, namely electrically connected to the battery, these child stations start their own operations under the "wake" conditions. Also, only when the respective child stations receive the sleep instruction signals issued from the parent station, are these child stations brought into the "sleep" conditions.
In the above-described conventional automobile multiplex communication system, if the power supply to the child station would be instantaneously interrupted, or the DC voltage of the power source (battery) would be varied due to a loose contact occurring in the power supply lines connected between the power source and the child stations operated, under sleep conditions, then the sleep conditions of the child stations would be mistakenly changed into the wake conditions, which is similar to such a condition that the child station is powered by the power source. If such wake conditions of the child stations are continued for a long time, then large consuming currents flow through the child stations. As a consequence, there is another problem that the power of the battery becomes highly consumed.
Furthermore, if the communication control microcomputer employed in the parent station would be operated under an extraordinary state, even when various information is transmitted from the child stations to this parent station, there are some risks that the information from the child stations could not be acquired by the parent station.